1. Field of the Invention
The subject disclosure relates to a fuel control system for use with gas turbine engines, and more particularly to, a fuel control system which utilizes computed signals from an embedded, real-time thermodynamic engine model when attempting to match actual core engine acceleration or deceleration rates to the demanded rate.
2. Background of the Related Art
Typically, a gas turbine engine control system modulates fuel flow to the engine in order to match or “track” the actual rate of change of the gas generator speed (NDOTActual) to the demanded rate of change of the gas generator speed, NDOTDemand. The maximum demanded NDOT rate is obtained from an acceleration schedule. The acceleration schedule is traditionally provided by the engine manufacturer and is developed over time to protect the engine from surge, stall and overtemperature. As a result, the acceleration schedule is specific or unique to a particular engine model. The schedule typically represents NDOTDemand as a function of measured gas generator speed (NH) and inlet air temperature and pressure. The schedule is not linear, but of complex shape. The complexity of the schedule is partly due to the need to prevent the engine from operating in the compressor stall region.
State-of-the-art digital control systems typically use a proportional plus integral (and sometimes derivative) NDOT control loop to modulate fuel flow and null out the error between the measured actual acceleration/deceleration rate of the core engine gas generator (NDOTActual) and the demanded rate (NDOTDemand). Since the engine is a highly non-linear complex machine, the matching or tracking of actual versus demanded NDOT rate is sometimes imperfect, especially during rapid engine accelerations or decelerations. More specifically, during severe operational transients, the control system is unable to drive the error between NDOTDemand and NDOTActual to zero.
The inability to track the NDOTActual rate to the NDOTDemand rate is partly caused by control design tradeoffs, namely bandwidth limitations which result from an overriding desire to insure control loop stability. More importantly however, current state-of-the-art control systems do not account for external disturbances to the NDOT control loop, such as real-time thermodynamic engine effects, which adversely affect NDOT rate tracking.
As a result of the inability to accurately track the actual NDOT rate to the demanded rate, engine surge events could occur if actual NDOT overshoots the acceleration limit. An engine surge creates a sudden torque disturbance to the driven load. In a helicopter application, an engine surge event typically imparts a torque disturbance to the load system, which consists generally of an engine output shaft, a clutch, a gearbox, and shaft driven main and tail rotors. The sudden torque disturbance can cause the underdamped rotor drive train to ring which can result in transient overstressing of mechanical parts and result in engine drive train damage.
Therefore, there is a need for an improved NDOT tracking system which during operational transients, more accurately matches the NDOTActual rate to the NDOTDemand rate by accounting for real-time thermodynamic engine effects.